This invention relates to an apparatus and method for controlling a multi-turbine installation. More specifically, the invention relates to controlling a plurality of turbine-driven systems, the combined output of which is predeterminedly apportioned between each of the contributing systems.
While not limited thereto, this invention has particular utility when utilized in conjunction with prime movers such as mechanical drive steam turbines. Mechanical drive turbines are used in industrial and utility applications for driving electrical generators, compressors, pumps and various other types of driven equipment, in combinations generally termed turbine-driven systems.
In one application, a mechanical drive turbine is connected to a feedpump for providing feedwater flow to a boiler. The boiler heats the feedwater to generate steam to power a power plant. In power plants having a relatively large boiler, several feedwater pumps may be required to meet the total feedwater requirement of the boiler. Each of these feedwater pumps is independently powered by a respective mechanical drive turbine. Such turbine-driven systems may be identically rated units operated for equally sharing a total system output requirement or they may be differently sized units having various capacity ratings and operated for proportionately sharing the total system output requirement. In each situation, however, it is desirable that a predetermined relationship of the individual outputs of the turbine-driven systems be maintained. Accordingly, apparatus that will provide for coordinating the operation of these independent turbine-driven systems for effecting load-sharing is necessary. One advantage of using such an apparatus is its effectiveness for subjecting the turbine-driven systems to equal wear at the point of going to overhaul in order not to have one wear out short of its rated time between overhauls. Each turbine-driven system should desirably produce a respective proportionate share of its rate capacity for equalizing wear.
In the application of a turbine-driven pump, the pump output flow is proportional to the rotor speed of the turbine. Electrical control systems for controlling rotor speed, and thereby pump output, by comparing actual speeds with reference signals in a closed loop are known in the art. Solid-state operational amplifiers have been used to provide the various desired functions in the form of electrical signals and the resulting amplified electrical signals have been used to operate servo-valves which, in turn, actuate hydraulic valve positioning rams to control the admission of steam to the turbines for controlling the speed thereof.
The customary method of adjusting turbine speeds for a plurality of turbines is to initially preset the respective reference rotor speeds for each turbine. However, due to physical tolerances in the turbines' feedpumps and piping systems, the output flow of each feedpump may be different even though turbine speeds are identical. In addition, physical tolerances may vary over the entire range of operation. During operation, external control means, not part of the present invention, determines the respective reference speeds for effecting a desired total combined system or installation output. Due to the fact that the relationship of system, or pump, output to rotor speed may change with time and operating conditions, it is difficult to accurately control the outputs of individual systems by controlling rotor speed alone. Accordingly, the systems must each be operated at below full rated output capacity to avoid the possibility of exceeding the rated output capacity of, or overloading, any of the turbine-driven equipment.